This invention relates to a semi-suspension polymerization process for producing polyblend polymeric composites and microcapsules with varying core-shell morphologies, and more particularly to a starved feed semi-suspension polymerization process. Such process is particularly useful in the production of in situ and conventional toner resins, and in the production of particles with a wide range of product properties.
The resins produced by the starved feed semisuspension polymerization process may be utilized for making dry electrostatographic toners and developers according to known methods. Such toners and developers may then be used in conventional electrostatographic imaging processes. The resins produced according to the described methods may also be used in liquid developers and inks, for example, for ink jet applications.
Conventional toner resins and methods for producing such resins are known in the art. Similarly, polyblend polymeric composites and methods for producing such composites are also known in the art. Such toner resins and polyblend composites may be produced by a variety of different methods. Such methods include suspension polymerization, solution polymerization, anionic polymerization, emulsion polymerization and batch emulsion polymerization.
Semi-suspension polymerization processes are known in the art. For example, U.S. Pat. Nos. 5,164,282, 5,236,629 and 5,288,585, the disclosures of which are totally incorporated herein by reference, disclose processes for preparing toner particles wherein at least one resin monomer is bulk polymerized with a polymerization initiator, a cross-linking component and a chain transfer agent until partial polymerization has been accomplished, and thereafter mixing pigments, dyes, conductive fillers, charge control agents, initiators, chain transfer agents, cross-linking agents or other particles with the partially polymerized product and thereafter heating the mixture to complete the suspension polymerization process. Also, U.S. Pat. No. 5,264,314, the disclosure of which is totally incorporated herein by reference, discloses a process for preparing toner compositions wherein at least one monomer is mixed with a polymerization initiator, a cross-linking component and a chain transfer agent, and the mixture is bulk polymerized until partial polymerization to near the onset of gel-effect occurs. Thereafter, the partially polymerized component is mixed with pigment or dye particles and a stabilizing component, and suspension polymerization is completed by heating the mixture. U.S. Pat. No. 5,225,279 discloses a two-stage process for forming microcapsules to encapsulate a solvent. The first and optional second stages are emulsion polymerizations of a monomer. In the optional second stage, a second monomer is added to the emulsion and is polymerized to further encapsulate the core material. However, none of these references disclose a starved feed semi-suspension polymerization process.
U.S. Pat. No. 5,306,593 discloses a starved feed semi-suspension polymerization process for producing polyblend toner particles comprised of resins with high molecular weight polymer domains dispersed in a low molecular weight polymer matrix. The reference does not disclose polyblend toner particles comprised of a resin with low molecular weight polymer domains dispersed in a high molecular weight polymer matrix, nor does it disclose the production of toner particles having core-shell, pseudo core-shell or inverted core-shell morphologies.
Other starved feed polymerization processes are also known in the art, although not as applied to the production of toner and developer compositions.
A discussion of theoretical and experimental studies of starved feed polymerization processes may be found in art publications. For example, a discussion of reaction kinetics focusing upon the monomer and initiator ratios may be found in O'Driscoll and Burczyk, "Kinetic Analysis of a Starved Feed Polymerization Reactor," Polymer Reaction Engineering, Vol. 1, No. 1, pp. 111-144 (1992), the full disclosure of which is incorporated herein by reference. Further kinetic analysis of semi-batch reaction processes, directed towards the possibility of explosive hazards due to thermal instability of such processes, is presented in Gray, Coppersthwaite and Griffiths, "A Novel, Thermal Instability in a `Semi-Batch` Reactor," Process Safety Progress, Vol. 12, No. 1, pp. 49-54 (1993), the full disclosure of which is incorporated herein by reference. Also, an experimental analysis of the critical feed time for starved feed addition may be found in Bourne and Thoma, "Some Factors Determining the Critical Feed Time of a Semi-Batch Reactor," Trans IChemE, Vol. 69, Part A. pp. 321-323 (1991), the full disclosure of which is incorporated herein by reference.
Recently, there has been tremendous growth in the quantity and variety of rubber-modified polyblend polymers. This growth is because the toughness of a material can be significantly enhanced by blending the material with a few percent of an elastomeric component. This approach is used, for example, with high impact polystyrene. However, conventional processes for producing such polyblend polymers possess several disadvantages and problems which reduce the usefulness of such processes in the toner/developer field. For example, some toners incorporate wax to improve the release properties of the toner composition. If the wax is not well dispersed in the toner, the wax will gradually accumulate in the imaging apparatus and contaminate the machine's subsystems. Further, polyblends are usually prepared by conventional processes of either melt-mixing or solution casting. Solution casting is time consuming and involves high cost of solvent handling, removal, treatment and disposal. Also, melt mixing of two incompatible polymers is very difficult, especially when the viscosities of the two polymers are very different. In order to achieve good dispersions of incompatible polymers, the conventional processes frequently require the addition of compatibilizers such as block or graft copolymers.
Another disadvantage of conventional methods for producing polyblend polymers is that the processes do not offer the flexibility to control particle morphology. As a result, the particles are typically uniform in composition. Although a uniform composition may be suitable for some applications, the uniformity considerably restricts the range of applications for which a particle material may be used.